Fix confusing button text when importing from GitHub

What does this MR do?

This MR fixes the text of the 'import all' button on the GitHub import page. When the user starts any import, the text of the "Import x repositories" button will change to "Importing..."

I think that it's better when the spinner remains. But I'm wondering whether we can only make the button loading/disabled when importAll is run (i.e. the "import x repositories" button is clicked and then the confirm button in the modal). Because technically, importAll should only run not started repository imports, right? (question @ reviewer)

Related to #285634 (closed)

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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